Improve performance on multiple versions (#13573)

Existing:

```go
type xlMetaV2 struct {
    Versions []xlMetaV2Version `json:"Versions" msg:"Versions"`
}
```

Serialized as regular MessagePack.

```go
//msgp:tuple xlMetaV2VersionHeader
type xlMetaV2VersionHeader struct {
	VersionID [16]byte
	ModTime   int64
	Type      VersionType
	Flags     xlFlags
}
```

Serialize as streaming MessagePack, format:

```
int(headerVersion)
int(xlmetaVersion)
int(nVersions)
for each version {
    binary blob, xlMetaV2VersionHeader, serialized
    binary blob, xlMetaV2Version, serialized.
}
```

xlMetaV2VersionHeader is <= 30 bytes serialized. Deserialized struct 
can easily be reused and does not contain pointers, so efficient as a 
slice (single allocation)

This allows quickly parsing everything as slices of bytes (no copy).

Versions are always *saved* sorted by modTime, newest *first*. 
No more need to sort on load.

* Allows checking if a version exists.
* Allows reading single version without unmarshal all.
* Allows reading latest version of type without unmarshal all.
* Allows reading latest version without unmarshal of all.
* Allows checking if the latest is deleteMarker by reading first entry.
* Allows adding/updating/deleting a version with only header deserialization.
* Reduces allocations on conversion to FileInfo(s).

cmd/xl-storage-format-utils_test.go

@@ -0,0 +1,93 @@
+package cmd
+
+import (
+	"testing"
+
+	xhttp "github.com/minio/minio/internal/http"
+)
+
+func Test_hashDeterministicString(t *testing.T) {
+	tests := []struct {
+		name string
+		arg  map[string]string
+	}{
+		{
+			name: "zero",
+			arg:  map[string]string{},
+		},
+		{
+			name: "nil",
+			arg:  nil,
+		},
+		{
+			name: "one",
+			arg:  map[string]string{"key": "value"},
+		},
+		{
+			name: "several",
+			arg: map[string]string{
+				xhttp.AmzRestore:                 "FAILED",
+				xhttp.ContentMD5:                 mustGetUUID(),
+				xhttp.AmzBucketReplicationStatus: "PENDING",
+				xhttp.ContentType:                "application/json",
+			},
+		},
+		{
+			name: "someempty",
+			arg: map[string]string{
+				xhttp.AmzRestore:                 "",
+				xhttp.ContentMD5:                 mustGetUUID(),
+				xhttp.AmzBucketReplicationStatus: "",
+				xhttp.ContentType:                "application/json",
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			const n = 100
+			want := hashDeterministicString(tt.arg)
+			m := tt.arg
+			for i := 0; i < n; i++ {
+				if got := hashDeterministicString(m); got != want {
+					t.Errorf("hashDeterministicString() = %v, want %v", got, want)
+				}
+			}
+			// Check casual collisions
+			if m == nil {
+				m = make(map[string]string)
+			}
+			m["12312312"] = ""
+			if got := hashDeterministicString(m); got == want {
+				t.Errorf("hashDeterministicString() = %v, does not want %v", got, want)
+			}
+			want = hashDeterministicString(m)
+			delete(m, "12312312")
+			m["another"] = ""
+
+			if got := hashDeterministicString(m); got == want {
+				t.Errorf("hashDeterministicString() = %v, does not want %v", got, want)
+			}
+
+			want = hashDeterministicString(m)
+			m["another"] = "hashDeterministicString"
+			if got := hashDeterministicString(m); got == want {
+				t.Errorf("hashDeterministicString() = %v, does not want %v", got, want)
+			}
+
+			want = hashDeterministicString(m)
+			m["another"] = "hashDeterministicStringhashDeterministicStringhashDeterministicStringhashDeterministicStringhashDeterministicStringhashDeterministicStringhashDeterministicString"
+			if got := hashDeterministicString(m); got == want {
+				t.Errorf("hashDeterministicString() = %v, does not want %v", got, want)
+			}
+
+			// Flip key/value
+			want = hashDeterministicString(m)
+			delete(m, "another")
+			m["hashDeterministicStringhashDeterministicStringhashDeterministicStringhashDeterministicStringhashDeterministicStringhashDeterministicStringhashDeterministicString"] = "another"
+			if got := hashDeterministicString(m); got == want {
+				t.Errorf("hashDeterministicString() = %v, does not want %v", got, want)
+			}
+
+		})
+	}
+}